1. Field of the Invention
The present invention relates to data transmission systems and, more particularly, to multicarrier data transmission systems.
2. Description of the Related Art
A conventional voice-band modem can connect computer users end-to-end through the Public Switched Telephone Network (PSTN). However, the transmission throughput of a voice-band modem is limited to below 40 Kbps due to the 3.5 KHz bandwidth enforced by bandpass filters and codes at the PSTN interface points. On the other hand, the twisted-pair telephone subscriber loop of a computer user has a much wider usable bandwidth. Depending on the length of the subscriber loop, the bandwidth at a loss of 50 dB can be as wide as 1 MHz. Transmission systems based on local subscriber loops are generally called Digital Subscriber Lines (DSL).
As consumer demand has increased for interactive electronic access to entertainment (e.g. video-on-demand) and information (Internet) in digital format, this demand has effectively exceeded the capabilities of conventional voice-band modems. Various delivery approaches have been proposed, such as optical fiber links to every home, direct satellite transmission, and wideband coaxial cable. However, these approaches are often too costly. Hence, cheaper alternatives have emerged, such as the cable modem which uses existing coaxial cable connections to homes and various high bit rate DSL modems which use the existing twisted-pair of copper wires connecting a home to the telephone company's central office (CO).
One DSL technique for high-speed data communications is Asymmetrical Digital Subscriber Line (ADSL) signaling for the telephone loop that has been defined by standards as a communication system specification that provides a low-rate data stream from the residence to the CO (upstream), and a high-rate data stream from the CO to the residence (downstream). The ADSL standard provides for operation without affecting conventional voice telephone communications, e.g., Plain Old Telephone Service (POTS). The ADSL upstream channel only provides simple control functions or low-rate data transfers. The high-rate downstream channel provides a much higher throughput. This asymmetrical information flow is desirable for applications such as video-on-demand (VOD).
An ADSL modem operates in a frequency range that is higher than the voice-band; this permits higher data rates. However, the twisted-pair subscriber line has distortion and losses which increase with frequency and line length. Thus, the ADSL standard data rate is determined by a maximum achievable rate for a length of subscriber lines.
The ADSL standard uses Discrete Multi-Tone (DMT) modulation with the DMT spectrum divided into two-hundred fifty-six 4.3125 KHz carrier bands and a quadrature amplitude modulation (QAM) type of constellation is used to load a variable number of bits onto each carrier band independently of the other carrier bands.
The number of bits per carrier is determined during a training period when a test signal is transmitted through the subscriber line to the receiving modem. Based on the measured signal-to-noise ratio of the received signal, the receiving modem determines the optimal bit allocation, placing more bits on the more robust carrier bands, and returns that information back to the transmitting modem.
The modulation of the coded bits is performed very efficiently by using a 512-point inverse fast Fourier transform (IFFT) to convert the frequency domain coded bits into a time domain signal which is put on the twisted-pair by a Digital-to-Analog (D/A) converter using a sample rate of 2.208 MHz (4.3125×512). The receiving ADSL modem samples the signal and recovers the coded bits with a fast Fourier transform (FFT).
A typical DMT system utilizes a transmitter inverse FFT and a receiver forward FFT. Ideally, the channel frequency distortion can be corrected by a frequency domain equalizer following the receiver FFT. However, the delay spread of the channel in the beginning of the receiver FFT block contains inter-symbol interference from the previous block. As this interference is independent of the current block of data, it cannot be canceled just by the frequency domain equalizer. The typical solution adds a block of prefix data in front of the FFT data block on the transmitter side before the block of FFT data is sent to the D/A converter. The prefix data is the repeat copy of the last section of FFT data block.
On the receiver side, the cyclic prefix is removed from the received signal. If the length of the channel impulse response is shorter than the prefix length, inter-symbol interference from the previous FFT data block is completely eliminated. Frequency domain equalizer techniques are then applied to remove intra-symbol interference among DMT subchannels. However, since the channel impulse response varies on a case by case basis, there is no guarantee that the length of the impulse response is shorter than the prefix length. An adaptive time domain equalizer is typically required to shorten the length of the channel response within the prefix length.
Time domain equalizer training procedures have been studied previously, see “Equalizer Training Algorithms for Multicarrier Modulation Systems,” J. S. Chow, J. M. Cioffi, and J. A. C. Bingham, 1993 International Conference on Communications, pages 761-765, Geneva, May 1993. A corresponding training sequence has also been specified in “ADSL standard and Recommended Training Sequence for Time domain Equalizers (TQE) with DMT,” J. S. Chow, J. M. Cioffi, and J. A. C. Bingham, ANSI T1E1.4 Committee Contribution number 93-086.
Besides ADSL, another DSL technique for high-speed data communications over twisted-pair phone lines is known as Very High Speed Digital Subscriber Lines (VDSL). VDSL is intended to facilitate transmission rates greater than that offered by the ADSL. The multi-carrier transmission schemes used with VDSL can be Discrete Multi-Tone (DMT) modulation, or some other modulation scheme such as Discrete Wavelet Multi-Tone (DWMT) modulation, Quadrature Amplitude Modulation (QAM), Carrierless Amplitude and Phase modulation (CAP), Quadrature Phase Shift Keying (QPSK), or vestigial sideband modulation.
A common feature of the above-mentioned transmission systems is that twisted-pair phone lines are used as at least a part of the transmission medium that connects a central office (e.g., telephone company) to users (e.g., a residence or business). Even though fiber optics may be available from a central office to the curb near a user's residence, twisted-pair phone lines are used to bring the signals from the curb into the user's home or business.
One conventional frame synchronization technique for a system, using frequency division duplexing (FDD) or echo cancelling to provide duplexed operation, required the transmission of a predetermined sequence of data which was received by a receiver and then correlated with a predetermined stored sequence of data to determine the adjustment required in order to yield synchronization. This frame synchronization technique requires a special start-up training sequence to obtain the frame synchronization.
When a data transmission system is operating in a time-division duplexed (TDD) manner, the transmitters and receivers of the central office and remote units must be synchronized in time so that transmission and reception do not overlap in time. In a data transmission system, downstream transmissions are from a central side transmitter to one or more remote side receivers, and upstream transmissions are from one or more remote side transmitters to a central side receiver. The central side transmitter and receiver can be combined as a central side transceiver, and the remote side transmitter and receiver can be combined as a remote side transceiver.
Generally speaking, in a time-division duplexed system, upstream signals are alternated with downstream signals. On channels subject to crosstalk (NEXT interference) between multiple connections, if time-division duplexing is used, synchronization must be established and maintained among all units so affected. Typically, the upstream transmissions and the downstream transmissions are separated by a guard interval or a quiet period. The guard interval is provided to enable the transmission system to reverse the direction in which data is being transmitted so that a transmission can be received before the transmission in the opposite direction occurs. Some transmission schemes divide upstream and downstream transmissions into smaller units referred to as frames. These frames may also be grouped into superframes that include a series of downstream frames and a series of upstream frames, as well as guard intervals between the two.
In multi-carrier data transmission systems, high speed data transfer can be performed using a plurality of sub-carriers. A Discrete Multi-Tone (DMT) symbol is transmitted using the plurality of sub-carriers. A cyclic prefix is inserted to maintain the circularity of DMT symbols. A cyclic prefix is formed by adding the last several samples to the beginning of a DMT symbol. The length of the cyclic prefix can be represented as Lcp. Likewise, a cyclic suffix can also be used for the same purpose and it is formed by adding the first few samples to the end of a DMT symbol. The length of the cyclic suffix can be represented as Lcs.
In addition, the cyclic suffix or cyclic prefix can be used for aligning the DMT transmit and receive windows in digital duplexing. See, e.g., John M. Cioffi et al. “G.vdsl: Digital Duplexing: VDSL Performance Improvement by Aversion of Frequency Guard Bands,” ITU Temporary Document NT-041, Nashville, Tenn., November 1999, which is incorporated herein by reference. Although the cyclic prefix is used as the mechanism to ensure circularity, the same principle applies to cyclic suffixes. Similarly, the cyclic suffix is used for aligning symbols in digital duplexing, but the same purpose can be fulfilled by either the cyclic prefix or the cyclic suffix, or a combination of both.
FIG. 1 is a diagram illustrating a representative, conventional DMT symbol 10 having a cyclic prefix 12 of length Lcp and a cyclic suffix 14 of length Lcs. A transmitter window is often applied to DMT symbols and the last β samples of the cyclic suffix of the preceding DMT symbol and the first β samples of the cyclic prefix of the latter DMT symbol are overlapped and added. The resulting length of the DMT symbol is thus defined asdmtsymbol_length=2*Nsc+Lcp+Lcs−β  (Eq. 1)where Nsc is the number of sub-carriers.
In determining the symbol boundary timing for multicarrier systems such as OFDM (orthogonal frequency division multiplexing—a generalization of DMT systems), a conventional method is described in L. Hanzo et al., “Single- and Multi-carrier Quadrature Amplitude Modulation,” Wiley, ISBN 0471492396, which is hereby incorporated herein by reference. By exploiting the cyclic prefix structure, a cross-correlation function is computed. The symbol delay index is chosen to maximize such a function. That is, the cross-correlation function, G(j), is formed with
                              G          ⁡                      (            j            )                          =                              ∑                          n              =              0                                      Lcp              -              1                                ⁢                                    r              ⁡                              (                                  j                  +                  n                                )                                      ·                          r              ⁡                              (                                  j                  +                  n                  +                                      2                    ·                                          N                      sc                                                                      )                                                                        (                  Eq          .                                          ⁢          2                )            where r refers to received samples. Further, the delay index, md, is selected to be x such that
                              G          ⁡                      (                          m              d                        )                          =                              max            x                    ⁢                                    {                              G                ⁡                                  (                  x                  )                                            }                        ⁢                                                  ⁢            for            ⁢                                                  ⁢            all            ⁢                                                  ⁢            x                                              (                  Eq          .                                          ⁢          3                )            
Accordingly, the conventional method seeks a symbol delay index to maximize the received signal energy. However, these high-speed data communication systems are subject to interference, e.g., inter-symbol interference and/or inter-carrier interference, which hinders operation. Inter-symbol interference is due to imperfections in the subscriber loop (i.e., non-ideal channel response), whereas inter-carrier interference is interference from one subcarrier to another. With longer loops or greater channel spread, the impact of such interference tends to worsen. Thus, there is always a need for improved approaches to determinate of appropriate symbol boundaries so that greater performance can be achieved.